1. Field of the Invention
The present invention relates to a magnetic tape having a large recording capacity suitable for use as a storage medium for office computers, including miniature computers and personal computers and for other computers for use in various applications, and in particular to a magnetic tape used as an external storage medium.
2. Description of the Related Art
Keeping pace with recent wide-spreading of office computers such as miniature computers and personal computers, there is a growing trend in research on a magnetic tape that serves as an external storage medium for recording computer data, which is known as a tape streamer. In the effort for putting the magnetic tape of this kind into practical use, there is a strong demand for increasing the recording capacity thereof in order to store a larger amount of data in a smaller body, corresponding with recent trends in computers such as down-sizing and enhancing information processing performance.
There are also growing demands on more advanced reliability of the magnetic tape which has been used under more diversified environmental conditions than before, in particular under wide variation of temperature and humidity conditions, and on more advanced performance of stable recording and reading of data under high-speed running of the tape repeated in a great number of times.
The magnetic tape generally has a non-magnetic support which is made of a flexible member of a synthetic resin or the like, and has a magnetic layer formed thereon. For the purpose of achieving a large recording capacity (volume recording capacity) sufficient for use in the above-described applications, it is generally believed as effective to use a ferromagnetic metal thin film as the magnetic layer to thereby raise the recording density of the magnetic layer per se, and to thin the total thickness of the magnetic tape. That is, a magnetic tape such that having a metal thin film formed on a non-magnetic support is beneficial, which tape is known as so-called evaporation tape.
A film most generally used for a non-magnetic support of the evaporation tape on which the magnetic metal thin film is formed is polyester film, which is mainly a polyethylene terephthalate film. In particular, polyethylene terephthalate film of approx. 7 to 10 μm thick is used as the non-magnetic support of home-video cassette tapes, which is typically an 8-mm tape, and polyethylene film of approx. 5 to 7 μm thick is used as that of tape streamer for computer data back-up.
One known strategy for elongating the recording time of a magnetic recording medium used for video tape is disclosed in Japanese Laid-Open Patent Publication No. 6-215350, which describes that the non-magnetic support is preferably composed of polyester as a major component, and more specifically of polyethylene naphthalate. There is, however, a further demand for thinning the recording medium in pursuit of still longer recording time of video cassette tapes and larger recording capacity of tape streamers.
With recent increase in information volume to be handled, there is a strong demand for increasing the recording capacity of tape streamers. This means shortening of the recording wavelength and narrowing of the track pitch. Both of such shortening of the recording wavelength and narrowing of the track pitch, however, may undesirably degrade the output level and S/N ratio. It is therefore necessary to further raise the performance of the head, and to further enhance the output level and S/N ratio.
There is known a major standard for tape storage called AIT (Advanced Intelligent Tape; names of a standard and a tape drive developed by Sony Corporation) which defines a format for tape streamers using 8-mm evaporation-type tape as a recording medium. With regard to the standard, the next-generation format AIT3 (approx. 100 GB/tape) has been coming up. Under AIT3, the recording capacity must further be upgraded as compared with that under the current AIT2 (approx. 50 GB/tape). Doubled capacity may be achieved by halving the conventional track pitch, but it also requires further enhancement of the output level higher than that under the current AIT2.
One known large-capacity magnetic tape is such that having an evaporated film of a ferromagnetic metal, such as Co, or an alloy thereof, and it is known that the capacity thereof can successfully be raised by introducing oxygen in the evaporation process for forming the magnetic metal thin film as the magnetic layer. A continuous take-up vacuum evaporation apparatus 10 as illustrated in FIG. 1 can be used in the method of depositing a metal magnetic material, which is typically Co, on the non-magnetic support while introducing oxygen. In the drawing, a non-magnetic support 1 is continuously fed out from a feeding roll 2, on the surface of which a metal magnetic material 3 (Co) is deposited under the presence of a trace amount of oxygen supplied from an oxygen introducing duct 4 so as to form a magnetic metal thin film, and is then taken up by a take-up roll 5. By introducing oxygen herein, size of Co crystal grown in the evaporation process is reduced to a very fine degree, and this increases residual magnetization Mr and thus improves the output level.